Add Cython as dependency required during installation (#177)

setup.py

@@ -148,7 +148,7 @@ def initialize_options(self):
     author_email='[email protected]',
     license='LGPL-3.0',
     python_requires='>=3',
-    install_requires=['numpy', 'mpi4py'],
+    install_requires=['numpy', 'mpi4py', 'Cython'],
     # mpi4py is only needed, if preCICE was compiled with MPI
     # see https://github.com/precice/python-bindings/issues/8
     packages=['precice'],